The present invention relates generally to semiconductor test equipment and, more specifically, to chucks used to hold wafers to be tested. Semiconductor manufacturers test semiconductor wafers throughout the manufacturing process but especially before they are processed to produce integrated circuits. The flattest wafers with the smoothest surface have been shown to yield the highest number of acceptable integrated circuits. Therefore, there is an increasing demand to produce test equipment that can detect microscopic defects in the wafers.
Integrated circuits are predominantly manufactured from 200 mm or 300 mm semiconductor wafers. The wafers incorporate a fiducial mark, either a flat in the normally circular edge or a small notch in the edge. These fiducials provide a reference point for identifying the position of measurements on the wafer surface. Typically the outer edge of the wafer surface, approximately a 3 mm wide exclusion band, is an area from which integrated circuits are not manufactured. The fiducials are contained within this exclusion band.
The desire to measure wafers with more precision and predictability leads to the need to support a wafer with minimal and predictable deformation while maximizing stiffness. Wafers are fragile, and can only be touched by materials that will not transfer contaminants to the surface. In addition, the equipment handling the wafer must not scratch it or permanently damage it. Vacuum chucks are well suited to the purpose, but the wafer is thin enough that an image of a chuck touching one surface will be discernible on the other surface.
Prior art test equipment has held wafers to be tested using chucks with a number of vacuum ports spaced beneath the wafers to hold them securely. Chucks designed for minimal vibration have included types displaying full backside contact or having pin contacts spaced regularly across the back. The chucks must be designed to introduce minimal errors into the wafer production and testing process. Errors may be introduced by contamination from chuck material contacting the wafer. A second error source is particle transfer from the surface of the chuck. In addition, particles already on the surface of the wafer may be forced into the surface by the contacting surfaces of the chuck. The force of the vacuum on the wafer will cause an effect termed print-through where the image of the contacting points of the chuck are detectable by measurement equipment on the opposite surface of the wafer. Prior art chucks have not succeeded in holding wafers securely without sacrificing a measure of contact contamination.
Once the wafer is securely held by the chuck, the test equipment rotates the wafer beneath a sensor to test the entire surface. The accuracy of measurements taken this way is limited by the stiffness of the wafer, the vibration of the wafer, the contamination of the surfaces, the amount of print through from the chuck and the instrument noise that cannot be factored out of the measurements taken. Very slow rotation speeds, that limit the test throughput, have been necessary to limit the effects of vibration.